1. Field of the Invention
The present invention relates to methods and apparatus for measuring internal information of scattering media, e.g., an absorption coefficient or a concentration of an absorber.
2. Related Background Art
Methods of measuring an absorption coefficient of a scattering medium being a medium to be measured, a concentration of an absorber or the like, based on the Microscopic Beer-Lambert Law (hereinafter referred to as xe2x80x9cMBL lawxe2x80x9d), include, for example, the methods disclosed in Japanese Patent Applications Laid-Open No. H08-94517, No. H10-73481, and No. H10-111238 by the inventors. These methods based on the MBL law possess the significant feature of being theoretically free of the influence of {circle around (1)} the shape of medium, {circle around (2)} boundary conditions, {circle around (3)} scattering, and so on, and the same analytical formulas can be applied to media having any medium shape, any boundary conditions, and various scattering characteristics as long as there is no reentry of photon into the scattering medium.
The measuring methods based on the MBL law can be roughly classified under four types at present. They are (1) Time Resolved Spectroscopy (hereinafter referred to as xe2x80x9cTRS methodxe2x80x9d) making use of the time-resolved profile of re-emission, (2) Time Integrated Spectroscopy (hereinafter referred to as xe2x80x9cTIS methodxe2x80x9d) making use of the time integral value of the time-resolved profile of re-emission and the mean pathlength, (3) Time Gating Spectroscopy (hereinafter referred to as xe2x80x9cTGS methodxe2x80x9d) making use of portions cut out of the time-resolved profile by gates, and (4) Phase Modulation Spectroscopy (hereinafter referred to as xe2x80x9cPMS methodxe2x80x9d) making use of modulated light. Among these, (2) the TIS method and (4) the PMS method utilizing all re-emission are advantageous from the viewpoint that optical attenuation due to scattering is great in living tissues and it is thus important to use as much re-emission as possible in practical use. These two measuring methods are in the relation of Fourier transform with each other.
While the measuring methods based on the MBL law have the many advantages as described above, the measurement accuracy thereof, however, is not sufficient yet for utilization and application in the wide range. For example, there was the problem that, in the case of a living tissue being a measured object, various individual differences, such as color of skin, presence/absence of hair, etc., affected the absolute value of light intensity or the like, so as to be the cause of degradation of the measurement accuracy. Further, the wavelength dependence of scattering coefficient also degrades the measurement accuracy.
There was another problem that reduction was insufficient in the computation time of analysis during measurement and it was thus difficult to implement real-time measurement.
The present invention has been accomplished in view of the above problems and an object of the invention is to provide measuring methods and apparatus of internal information of scattering medium capable of measurement with higher accuracy and at higher speed, when compared with the conventional measuring methods based on the MBL law.
The inventor has conducted intensive and extensive research in order to accomplish the above object and finally found that it became feasible to implement highly accurate measurement by measuring the mean pathlength and variance, particularly, for pulsed light of a plurality of different wavelength components, by the Mean and Variance based Spectroscopy (hereinafter referred to as xe2x80x9cMVS methodxe2x80x9d) utilizing the mean pathlength and variance, or physical quantities equivalent thereto, without use of information such as absolute values of light intensity or a ratio thereof, and to readily put the wavelength dependence of scattering coefficient into analytical formulas, thus accomplishing the present invention. Likewise, the inventor found that it also became feasible to implement highly accurate measurement by measuring the group delay for modulated light of a plurality of different wavelength components and the second partial derivative of logarithm of amplitude with respect to modulation frequency, thus accomplishing the present invention.
Specifically, a first measuring method of internal information of a scattering medium according to the present invention is a method comprising (1) a light injecting step of injecting pulsed light of two or more predetermined wavelengths into a scattering medium at a light injection position, (2) a light detecting step of detecting the light of the two or more predetermined wavelengths having propagated inside the scattering medium, at a photodetection position to acquire a photodetection signal, (3) a signal processing step of acquiring waveform data indicating a temporal change of intensity of the detected light, based on the photodetection signal, (4) a mean pathlength and variance computing step of performing an operation to compute a mean pathlength of plural photons composing the detected light, and a variance, based on the waveform data, and (5) an absorption coefficient difference calculating step of calculating a difference between absorption coefficients at the predetermined wavelengths, based on a predetermined relation holding among the mean pathlength, the variance, and the difference between the absorption coefficients at the two or more predetermined wavelengths.
A first measuring apparatus of internal information of a scattering medium according to the present invention is an apparatus comprising (1) light injecting means for injecting pulsed light of two or more predetermined wavelengths into a scattering medium at a light injection position, (2) light detecting means for detecting the light of the two or more predetermined wavelengths having propagated inside the scattering medium, at a photodetection position to acquire a photodetection signal, (3) signal processing means for acquiring waveform data indicating a temporal change of intensity of the detected light, based on the photodetection signal, (4) mean pathlength and variance computing means for performing an operation to compute a mean pathlength of plural photons composing the detected light, and a variance, based on the waveform data, and (5) absorption coefficient difference calculating means for calculating a difference between absorption coefficients at the predetermined wavelengths, based on a predetermined relation holding among the mean pathlength, the variance, and the difference between the absorption coefficients at the two or more predetermined wavelengths.
The above first method and apparatus according to the present invention are based on a TIMVS method, which is an MVS method in which the analysis is carried out in the time domain by the time integrated spectroscopy (TIS method). The TIMVS method using the mean pathlength and variance for the light of plural wavelength components in this way presents very significant advantages in practical use, including {circle around (1)} being not influenced by the individual differences and absolute values of intensity of incident light dependent upon wavelengths and positions, {circle around (2)} extremely simplifying the quantitative formula involving the wavelength dependence of scattering coefficient, {circle around (3)} decreasing errors in determination of the zero point (t =0) of the time axis in the time-resolved spectroscopy, and so on, in addition to the advantages presented by the conventional measuring methods based on the MBL law.
In the method and apparatus, it also becomes feasible to greatly reduce the measurement and analysis time and implement real-time measurement, by applying the Simple Subtraction Method (SSM) developed by the inventors, which permits the mean pathlength of detected photons (the center of gravity of a time-resolved profile) and the variance to be computed at high speed by calculating the moment of the time-resolved profile with a computer. This SSM is described, for example, in Japanese Patent Application Laid-Open No. H09-61343.
A second measuring method of internal information of a scattering medium according to the present invention is a method comprising (1) a light injecting step of injecting modulated light of two or more predetermined wavelengths modulated at a predetermined frequency, into a scattering medium at a light injection position, (2) a light detecting step of detecting the light of the two or more predetermined wavelengths having propagated inside the scattering medium, at a photodetection position to acquire a photodetection signal, (3) a signal processing step of extracting a signal of the predetermined frequency component from the photodetection signal, (4) a group delay and second-partial-derivative-of-logarithm-of-amplitude computing step of computing a group delay of the signal of the predetermined frequency component and a second partial derivative of logarithm of amplitude with respect to the modulation frequency, based on the signal of the predetermined frequency component, and (5) an absorption coefficient difference calculating step of calculating a difference between absorption coefficients at the predetermined wavelengths, based on a predetermined relation holding among the group delay, the second partial derivative of logarithm of amplitude with respect to the modulation frequency, and the difference between the absorption coefficients at the two or more predetermined wavelengths.
A second measuring apparatus of internal information of a scattering medium according to the present invention is an apparatus comprising (1) light injecting means for injecting modulated light of two or more predetermined wavelengths modulated at a predetermined frequency, into a scattering medium at a light injection position, (2) light detecting means for detecting the light of the two or more predetermined wavelengths having propagated inside the scattering medium, at a photodetection position to acquire a photodetection signal, (3) signal processing means for extracting a signal of the predetermined frequency component from the photodetection signal, (4) group delay and second-partial-derivative-of-logarithm-of-amplitude computing means for computing a group delay of the signal of the predetermined frequency component and a second partial derivative of logarithm of amplitude with respect to the modulation frequency, based on the signal of the predetermined frequency component, and (5) absorption coefficient difference calculating means for calculating a difference between absorption coefficients at the prescribed wavelengths, based on a predetermined relation holding among the group delay, the second partial derivative of logarithm of amplitude with respect to the modulation frequency, and the difference between the absorption coefficients at the two or more predetermined wavelengths.
The above-described second method and apparatus according to the present invention are based on a PMMVS method, which is an MVS method in which the analysis is carried out in the frequency domain by the phase modulation spectroscopy (PMS method). This PMMVS method is in the relation of Fourier transform with the TIMVS method associated with the first method and apparatus according to the present invention, and the PMMVS method using the group delay and the second partial derivative of logarithm of amplitude with respect to the modulation frequency for the light of the plural wavelengths in this way presents very significant advantages in practical use, including the aforementioned advantages {circle around (1)}, {circle around (2)} described as to the TIMVS method, and {circle around (4)} decreasing errors in determination of the zero point of the phase in the phase modulation spectroscopy, and so on, in addition to the advantages provided by the conventional measuring methods based on the MBL law.